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  • 1.
    Boström, Mathias
    et al.
    Department of Energy and Process Engineering, Norwegian University of Science and Technology, Norway.
    Ellingsen, Simen Ådnöj
    Department of Energy and Process Engineering, Norwegian University of Science and Technology, Norway.
    Brevik, Iver
    Department of Energy and Process Engineering, Norwegian University of Science and Technology, Norway.
    Dou, M. F.
    Department of Materials Science and Engineering, Royal Institute of Technology, Sweden.
    Persson, Clas
    Department of Physics, University of Oslo, Norway.
    Sernelius, Bo E.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska högskolan.
    Casimir attractive-repulsive transition in MEMS2012Ingår i: European Physical Journal B: Condensed Matter Physics, ISSN 1434-6028, E-ISSN 1434-6036, Vol. 85, nr 11Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Unwanted stiction in micro- and nanomechanical (NEMS/MEMS) systems due to dispersion (van der Waals, or Casimir) forces is a significant hurdle in the fabrication of systems with moving parts on these length scales. Introducing a suitably dielectric liquid in the interspace between bodies has previously been demonstrated to render dispersion forces repulsive, or even to switch sign as a function of separation. Making use of recently available permittivity data calculated by us we show that such a remarkable non-monotonic Casimir force, changing from attractive to repulsive as separation increases, can in fact be observed in systems where constituent materials are in standard NEMS/MEMS use requiring no special or exotic materials. No such nonmonotonic behaviour has been measured to date. We calculate the force between a silica sphere and a flat surface of either zinc oxide or hafnia, two materials which are among the most prominent for practical microelectrical and microoptical devices. Our results explicate the need for highly accurate permittivity functions of the materials involved for frequencies from optical to far-infrared frequencies. A careful analysis of the Casimir interaction is presented, and we show how the change in the sign of the interaction can be understood as a result of multiple crossings of the dielectric functions of the three media involved in a given set-up.

  • 2.
    Boström, Mathias
    et al.
    Royal Institute of Technology, Stockholm, Sweden, EU.
    Persson, Clas
    University of Oslo, Norway.
    Sernelius, Bo E.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska högskolan.
    Casimir force between atomically thin gold films2013Ingår i: European Physical Journal B: Condensed Matter Physics, ISSN 1434-6028, E-ISSN 1434-6036, Vol. 86, s. 43-46Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We have used density functional theory to calculate the anisotropic dielectric functions forultrathin gold sheets (composed of 1, 3, 6, and 15 atomic layers). Such films are important components innano-electromechanical systems. When using correct dielectric functions rather than bulk gold dielectricfunctions we predict an enhanced attractive Casimir-Lifshitz force (at most around 20%) between twoatomically thin gold sheets. For thicker sheets the dielectric properties and the corresponding Casimirforces approach those of gold half-spaces. The magnitude of the corrections that we predict should, withinthe today’s level of accuracy in Casimir force measurements, be clearly detectable.

  • 3.
    de Almeida Junior, E. F.
    et al.
    University of Federal Bahia, Brazil.
    de Brito Mota, F.
    University of Federal Bahia, Brazil.
    de Castilho, C. M. C.
    University of Federal Bahia, Brazil.
    Kakanakova-Gueorgieva, Anelia
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska högskolan.
    Gueorguiev, Gueorgui Kostov
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska högskolan.
    Defects in hexagonal-AlN sheets by first-principles calculations2012Ingår i: European Physical Journal B: Condensed Matter Physics, ISSN 1434-6028, E-ISSN 1434-6036, Vol. 85, nr 1Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Theoretical calculations focused on the stability of an infinite hexagonal AlN (h-AlN) sheet and its structural and electronic properties were carried out within the framework of DFT at the GGA-PBE level of theory. For the simulations, an h-AlN sheet model system consisting in 96 atoms per super-cell has been adopted. For h-AlN, we predict an Al-N bond length of 1.82 angstrom and an indirect gap of 2.81 eV as well as a cohesive energy which is by 6% lower than that of the bulk (wurtzite) AlN which can be seen as a qualitative indication for synthesizability of individual h-AlN sheets. Besides the study of a perfect h-AlN sheet, also the most typical defects, namely, vacancies, anti-site defects and impurities were also explored. The formation energies for these defects were calculated together with the total density of states and the corresponding projected states were also evaluated. The charge density in the region of the defects was also addressed. Energetically, the anti-site defects are the most costly, while the impurity defects are the most favorable, especially so for the defects arising from Si impurities. Defects such as nitrogen vacancies and Si impurities lead to a breaking of the planar shape of the h-AlN sheet and in some cases to the formation of new bonds. The defects significantly change the band structure in the vicinity of the Fermi level in comparison to the band structure of the perfect h-AlN which can be used for deliberately tailoring the electronic properties of individual h-AlN sheets.

  • 4.
    Garhammer, Julian
    et al.
    Univ Bayreuth, Germany.
    Hofmann, Fabian
    Univ Bayreuth, Germany.
    Armiento, Rickard
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska fakulteten.
    Kuemmel, Stephan
    Univ Bayreuth, Germany.
    On the challenge to improve the density response with unusual gradient approximations2018Ingår i: European Physical Journal B: Condensed Matter Physics, ISSN 1434-6028, E-ISSN 1434-6036, Vol. 91, nr 7, artikel-id 159Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Certain excitations, especially ones of long-range charge transfer character, are poorly described by time-dependent density functional theory (TDDFT) when typical (semi-)local functionals are used. A proper description of these excitations would require an exchange-correlation response differing substantially from the usual (semi-) local one. It has recently been shown that functionals of the generalized gradient approximation (GGA) type can yield unusual potentials, mimicking features of the exact exchange derivative discontinuity and showing divergences on orbital nodal surfaces. We here investigate whether these unusual potential properties translate into beneficial response properties. Using the Sternheimer formalism we closely investigate the response obtained with the 2013 exchange approximation by Armiento and Kummel (AK13) and the 1988 exchange approximation by Becke (B88), both of which show divergences on orbital nodal planes. Numerical calculations for Na-2 as well as analytical and numerical calculations for the hydrogen atom show that the response of AK13 behaves qualitatively different from usual semi-local functionals. However, the AK13 functional leads to fundamental instabilities in the asymptotic region that prevent its practical application in TDDFT. Our findings may help the development of future improved functionals. They also corroborate that the frequency-dependent Sternheimer formalism is excellently suited for running and analyzing TDDFT calculations.

  • 5.
    Johansson, Magnus
    et al.
    Linköpings universitet, Tekniska högskolan. Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik.
    Morgante, A.M.
    Laboratoire Léon Brillouin, CEA Saclay, 91191 Gif-sur-Yvette Cedex, France.
    Aubry, S.
    Laboratoire Léon Brillouin, CEA Saclay, 91191 Gif-sur-Yvette Cedex, France.
    Kopidakis, G.
    Department of Physics, University of Crete, PO Box 2208, 71003, Heraklion, Crete, Greece.
    Standing wave instabilities, breather formation and thermalization in a Hamiltonian anharmonic lattice2002Ingår i: European Physical Journal B: Condensed Matter Physics, ISSN 1434-6028, E-ISSN 1434-6036, Vol. 29, nr 2, s. 279-283Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Modulational instability of travelling plane waves is often considered as the first step in the formation of intrinsically localized modes (discrete breathers) in anharmonic lattices. Here, we consider an alternative mechanism for breather formation, originating in oscillatory instabilities of spatially periodic or quasiperiodic nonlinear standing waves (SWs). These SWs are constructed for Klein-Gordon or Discrete Nonlinear Schrödinger lattices as exact time periodic and time reversible multibreather solutions from the limit of uncoupled oscillators, and merge into harmonic SWs in the small-amplitude limit. Approaching the linear limit, all SWs with nontrivial wave vectors (0 < Q < p) become unstable through oscillatory instabilities, persisting for arbitrarily small amplitudes in infinite lattices. The dynamics resulting from these instabilities is found to be qualitatively different for wave vectors smaller than or larger than p/2, respectively. In one regime persisting breathers are found, while in the other regime the system thermalizes.

  • 6.
    Ou, Haiyan
    et al.
    Technical University of Denmark, Lyngby, Denmark .
    Ou, Yiyu
    Technical University of Denmark, Lyngby, Denmark .
    Argyraki, Aikaterini
    Technical University of Denmark, Lyngby, Denmark .
    Schimmel, Saskia
    University of Erlangen-Nuremberg, Erlangen, Germany .
    Kaiser, Michl
    University of Erlangen-Nuremberg, Erlangen, Germany .
    Wellmann, Peter
    University of Erlangen-Nuremberg, Erlangen, Germany .
    Linnarsson, Margareta
    Jokubavicius, Valdas
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska högskolan.
    Sun, Jianwu
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska högskolan.
    Liljedahl, Rickard
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska högskolan.
    Syväjärvi, Mikael
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska högskolan.
    Advances in wide bandgap SiC for optoelectronics2014Ingår i: European Physical Journal B: Condensed Matter Physics, ISSN 1434-6028, E-ISSN 1434-6036, Vol. 87, s. 58-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Silicon carbide (SiC) has played a key role in power electronics thanks to its unique physical properties like wide bandgap, high breakdown field, etc. During the past decade, SiC is also becoming more and more active in optoelectronics thanks to the progress in materials growth and nanofabrication. This paper will review the advances in fluorescent SiC for white light-emitting diodes, covering the poly-crystalline doped SiC source material growth, single crystalline epitaxy growth of fluorescent SiC, and nanofabrication of SiC to enhance the extraction efficiency for fluorescent SiC based white LEDs.

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